The invention relates to a telescopic riser section device, more particularly said riser section being provided with at least one actuator arranged to apply a downward tensile force to the riser, an actuator-pressurizing circuit being connected to the at least one actuator and being arranged on the riser section and/or on the riser.
Risers of this kind normally form a connection between a subsea well and a surface vessel, a number of conduits and pipes being extended between the well and the surface vessel. At its lower end, the riser is fitted to subsea equipment, such as blowout preventer valves, wellheads or similar, and at its upper end, it is connected to the surface vessel, for example a drillship or a platform.
The riser must continuously be kept under tension, and this is normally achieved by so-called heave compensators arranged on the surface vessel, steel ropes attached to the riser being kept taut by means of winches or hydraulic/pneumatic cylinders provided with pressure sources and accumulators. It is also known to use hydraulic/pneumatic cylinders directly, that is without any steel ropes. The heave compensating system must be dimensioned to take up the weight of the riser and any fluid inside it. Moreover, the system must be controllable to provide the so-called heave compensation, that is to say the vertical wave motion is compensated, so that the heave movements of the surface vessel are transferred to the riser to the least possible extent.
To be able to maintain a continuous riser connection between the well and vessel also during the vertical heave movements of the vessel, it is known to provide the upper end portion of the riser with a telescopic pipe section. It is also known to arrange the telescopic pipe section at another portion of the riser.
On interruption of a borehole operation, it may be relevant to pull the surface vessel away from the well, by shutting off the well and disconnecting the riser from the wellhead.
From U.S. Pat. No. 4,557,332 is known a riser with ballast units which provide some buoyancy in the riser. At the attachment of the riser to the surface vessel are arranged means which are arranged to pull the entire riser upwards, so that the lower end portion of the riser achieves a safe distance to the wellhead.
During operations at great depths and with correspondingly long risers, the heave-compensating suspension device will require a considerable lifting capacity because of the large mass of the riser, which complicates the surface vessel and increases its cost. For that reason, it may be appropriate to arrange the telescopic pipe section at the lower end portion of the riser, the riser being suspended directly from the surface vessel without any form of heave compensation.
From NO 308379 is known a riser which extends between a piece of subsea equipment and a surface vessel, wherein the riser is provided with a telescopic section at the lower end of the riser, heave compensation being effected by the telescopic movability of the riser, whereas, by such suspension, the mass of the riser keeps the riser under tension. There are also described means for pre-tensioning the telescopic section by a flange, arranged on the inner pipe and enclosed by the outer telescoping pipe, being arranged to be pressure-loaded for pre-tensioning purposes by means of the water pressure and/or by the use of spring force.